1. Technical Field of the Invention
The present invention relates generally to wavelength selectivity in broad band emission lasers and more particularly to an improved multiple plate stack for a birefringent filter to achieve such wavelength selectivity.
2. Discussion of the Related Art
Some lasers, notably dye lasers and some solid-state lasers, offer wide frequency tunability. For many applications such as atmospheric remote sensing, a single well defined wavelength within the continuous spectral output range must be selected. Unless some type of wavelength selective element is provided with wide band lasers, the laser will operate across a wide wavelength interval. Wavelength selective elements currently in use include birefringent filters, prisms, etalons, and gratings. Techniques involving the use of these conventional elements are well developed and many commercially available lasers are supplied with one or more line narrowing/wavelength selective components.
The tuning range of current birefringent filter designs is limited by the thickness of the thinnest plate, which must exceed a few hundred micrometers to maintain good optical quality. Conventional birefringent filters capable of spanning the entire tuning range of emerging wide band vibronic solid-state lasers require fabrication of extremely thin plates which could result in questionable optical quality.
The typical birefringent filter consists of several plates fabricated from an optical material that exhibits double refraction or birefringence. Polarized light incident on one surface of a single plate is split into ordinary and extraordinary components which travel through the material at different velocities. When these components recombine on the opposite surface of the plate, the net difference between the phases of the ordinary and extraordinary rays causes the resultant beam to have a different polarization from the incoming beam. Because losses depend on angles and polarizations, the emerging beam also has a different amplitude from the incoming beam.
Wavelength selectivity is possible with the typical birefringent filter because the outgoing beam of light is identical to the incoming beam at some values of the net phase difference. See, e.g., U.S. Pat. No. 4,569,053 to Roullard, III, et al. A series of plates having different thicknesses give the necessary flexibility to achieve high losses at all but a few wavelengths. These highly transmitted wavelengths are tunable by varying the angle of the incoming beam with respect to the optical axis of the material. The thicknesses of the thicker plates are normally integral multiples of the thickness of the thinnest plate. The usual method of increasing the wavelength separation between the highly transmitted waves is to fabricate thinner plates, which leads to a degradation in optical quality as discussed above.